How can I clone part of the texture?
To use the clone brush :


Select the clone brush in the paint tools
Ctrl+LMB to set the origin of what to clone from (the 3D cursor is set at this origin)
Then simply paint on the target position


Painting with a strength set to 1 is often better, but it depends on what you want.



If you want to clone from a picture that is not in the current model, you may need to set this picture on another mesh then join the two meshes, as this tool does not handle to clone parts that are outside of the painted mesh.